Modern electrical therapeutic and diagnostic devices, such as pacemakers or nerve stimulators for example, require a reliable electrical connection between the device and the body. In cases of nerve stimulators, in particular, chronically reliable electrical connections have been diffficult to attain. In a chronic setting it has been found many medical electrical leads may damage a nerve either mechanically or electrically or both.
Mechanically induced damage includes thickened epineurium due to accumulation of connective tissue between the electrode and the nerve, increased subperineural and endoneural connective tissue, endoneural edema, demyelinization, axonal degeneration and frank axonal loss. Such damage may be caused in several ways. First, if the lead and in particular the electrode which interfaces with the nerve does not move with the nerve, then abrasion may result. Second, the presence of the lead and in particular the electrode, a foreign object, may cause edema or swelling of the nerve. As the nerve swells it may be constricted by the lead. A compressive force may thereby be induced upon the nerve. In the past a so called "self-sizing" nerve electrode was fashioned to avoid such damage. Such an electrode may be seen in the U.S. Pat. No. 4,920,979 to Bullara entitled "Bidirectional Helical Electrode for Nerve Stimulation and assigned to the Huntington Medical Research Institute. To date, however, such electrodes have not been wholly satisfactory.
Electrically induced damage may also be caused by a chronic nerve electrode. Such damage results in, among other injuries, axonal degeneration as well as nerve edema. While it has been shown that the type of electrical stimulation, e.g. frequency, waveform, amplitude, may be a significant factor, the actual electrode design may also affect the degree of electrically induced damage. In particular a medical lead which provides the optimal electrical characteristics for the desired therapy is needed.